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Showing papers on "Palladium published in 2012"


Journal ArticleDOI
TL;DR: The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials.
Abstract: Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references).

1,899 citations


Journal ArticleDOI
TL;DR: PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes.
Abstract: Palladium-catalyzed cross-coupling reactions enable organic chemists to form C-C bonds in targeted positions and under mild conditions. Although phosphine ligands have been intensively researched, in the search for even better cross-coupling catalysts attention has recently turned to the use of N-heterocyclic carbene (NHC) ligands, which form a strong bond to the palladium center. PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes. This Review shows the success of these species in Suzuki-Miyaura, Negishi, and Stille-Migita cross-couplings as well as in amination and sulfination reactions.

740 citations


Journal ArticleDOI
09 Mar 2012-Science
TL;DR: It is shown that alumina (Al2O3) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts.
Abstract: We showed that alumina (Al(2)O(3)) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours The yield of ethylene was improved on all ALD Al(2)O(3) overcoated Pd catalysts

676 citations


Journal ArticleDOI
12 Apr 2012-Nature
TL;DR: Recent advances involving both the fundamental chemistry and the applications of these high-valent metal complexes in numerous synthetically useful catalytic transformations are described.
Abstract: Advances in the chemistry of high-oxidation-state copper and palladium demonstrate their great potential as catalysts. Homogeneous copper and palladium catalysts are used to synthesize a broad range of organic molecules, including pharmaceuticals, commodity chemicals and polymers. Improvements in the activity, selectivity and scope of these catalysts have the potential to increase their usefulness and reduce the environmental impact of the chemical reactions. In this Review, Amanda Hickman and Melanie Sanford summarize recent advances by organometallic chemists working with a subset of catalysts that operate through an intermediate containing palladium in the +3 or +4 oxidation state or copper in the +3 oxidation state. Copper and palladium catalysts are critically important in numerous commercial chemical processes. Improvements in the activity, selectivity and scope of these catalysts could drastically reduce the environmental impact, and increase the sustainability, of chemical reactions. One rapidly developing strategy for achieving these goals is to use ‘high-valent’ organometallic copper and palladium intermediates in catalysis. Here we describe recent advances involving both the fundamental chemistry and the applications of these high-valent metal complexes in numerous synthetically useful catalytic transformations.

669 citations


Journal ArticleDOI
TL;DR: In this paper, a carbon-supported Co3O4 electrocatalyst with nano-rods and spherical structures is reported for the oxygen reduction reaction (ORR) in alkaline media.
Abstract: We report preparation of carbon-supported Co3O4 electrocatalysts with nano-rods and spherical structures by the solvent-mediated morphological control method. The catalytic properties of the prepared catalysts for the oxygen reduction reaction (ORR) in alkaline media are investigated. We show that the ORR catalytic activity of the prepared catalysts is sensitive to the number and activity of surface-exposed Co3+ ions that can be tailored by the morphology of cobalt oxides. In particular, we demonstrate that the non-precious Co3O4 electrocatalyst with the nano-rod structure (∼12 nm in length and ∼5.1 nm in diameter) prepared in the mixed solvent of water to dimethylformamide ratio of 1 : 1 exhibits a higher current density than a much more expensive palladium-based catalyst does at the low potential region.

481 citations


Journal ArticleDOI
TL;DR: It is found that WC and W(2)C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements.
Abstract: This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W2C) and molybdenum carbide (Mo2C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W2C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt–WC and Pt–W2C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is ...

452 citations


Journal ArticleDOI
TL;DR: In this article, a discussion of cross-coupling chemistry of these two metals can be performed within a common mechanistic paradigm, helping to elucidate the key factors governing the behavior of the transition-metal complexes involved.

355 citations


Journal ArticleDOI
TL;DR: The focus of this Review is on complexes that should, in principle, exist as discrete molecular species in solution, and which are therefore of interest for their reactivity, their future synthetic utility and potential applications, for example, in catalysis or nanoscience.
Abstract: Polyoxometalates containing noble metal ions, such as ruthenium, osmium, rhodium, palladium, platinum, silver and gold, are a structurally diverse class of compounds. They include both classical heteropolyanions (vanadates, molybdates, tungstates) in which noble metals are present as heteroatoms, as well as the recently discovered class of polyoxometalates with noble metal "addenda" atoms. The focus of this Review is on complexes that should, in principle, exist as discrete molecular species in solution, and which are therefore of interest for their reactivity, their future synthetic utility and potential applications, for example, in catalysis or nanoscience.

339 citations


Journal ArticleDOI
TL;DR: In this article, a single layer graphene decorated with palladium (Pd) nanoparticles is fabricated using a single-layer H 2 sensor, which is able to detect as low as 20ppm H 2 at room temperature (22°C).
Abstract: Flexible hydrogen gas (H 2 ) sensors are fabricated using a single layer graphene decorated with palladium (Pd) nanoparticles. Thermally evaporated Pd is generally deposited on a graphene in the form of nanoparticles when the deposition thickness is very small. The graphene sensor with Pd thickness of 3 nm exhibits a gas response of ∼33% when exposed to 1000 ppm H 2 and it is able to detect as low as 20 ppm H 2 at room temperature (22 °C). The sensor is so flexible that any significant degradation is not observed when it is bent to a curved geometry with a bending radius of 3 mm. The flexible hydrogen sensors are applicable to a broad range of systems with demanding mechanical flexibility, durability and high gas response.

274 citations


Journal ArticleDOI
TL;DR: In this paper, a large number of immobilized-Pd-catalysts for cross-coupling reactions have been introduced in the last decade, and the observed catalyzed reactions truly heterogeneous or are they homogeneous due to leached palladium?
Abstract: A large number of immobilized-Pd-catalysts for cross-coupling reactions have been introduced in the last decade. Are the observed catalyzed reactions truly heterogeneous or are they homogeneous due to leached palladium? This account critically addresses the leaching issue by selectively referring to some of the newly developed catalytic systems in an attempt to evaluate said systems based on uniform criteria. The report is concluded by identifying the relevant chemical and structural challenges in the field.

252 citations


Journal ArticleDOI
TL;DR: In this paper, a series of carbon-supported Pd-Sn binary alloyed catalysts were used as anode electrocatalysts for direct ethanol fuel cell reactions in an alkaline medium.
Abstract: In this paper, we present a study of a series of carbon-supported Pd–Sn binary alloyed catalysts prepared through a modified Polyol method as anode electrocatalysts for direct ethanol fuel cell reactions in an alkaline medium. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and aberration-corrected scanning transmission electron microscopy equipped with electron energy loss spectroscopy were used to characterize the Pd–Sn/C catalysts, where homogeneous Pd–Sn alloys were determined to be present with the surface Sn being partially oxidized. Among various Pd–Sn catalysts, Pd86Sn14/C catalysts showed much enhanced current densities in cyclic voltammetric and chronoamperometric measurements, compared to commercial Pd/C (Johnson Matthey). The overall rate law of ethanol oxidation reaction for both Pd86Sn14/C and commercial Pd/C were also determined, which clearly showed that Pd86Sn14/C was more favorable in high ethanol concentration ...

Journal ArticleDOI
08 May 2012-Langmuir
TL;DR: A nexus between the catalyst support and catalyst particles is believed to yield the high hydrogen uptake capacities obtained.
Abstract: A high hydrogen storage capacity for palladium decorated nitrogen-doped hydrogen exfoliated graphene nanocomposite is demonstrated under moderate temperature and pressure conditions. The nitrogen doping of hydrogen exfoliated graphene is done by nitrogen plasma treatment, and palladium nanoparticles are decorated over nitrogen-doped graphene by a modified polyol reduction technique. An increase of 66% is achieved by nitrogen doping in the hydrogen uptake capacity of hydrogen exfoliated graphene at room temperature and 2 MPa pressure. A further enhancement by 124% is attained in the hydrogen uptake capacity by palladium nanoparticle (Pd NP) decoration over nitrogen-doped graphene. The high dispersion of Pd NP over nitrogen-doped graphene sheets and strengthened interaction between the nitrogen-doped graphene sheets and Pd NP catalyze the dissociation of hydrogen molecules and subsequent migration of hydrogen atoms on the doped graphene sheets. The results of a systematic study on graphene, nitrogen-doped g...

Journal ArticleDOI
TL;DR: In this paper, the experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach were discussed.
Abstract: Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively.


Journal ArticleDOI
TL;DR: A highly active, reusable, self-assembled catalyst of poly(imidazole-acrylamide) and palladium species inspired by metalloenzymes is presented and the convolution methodology to the preparation of polymeric metal catalysts is applied.
Abstract: Metalloenzymes are essential proteins with vital activity that promote high-efficiency enzymatic reactions. To ensure catalytic activity, stability, and reusability for safe, nontoxic, sustainable chemistry, and green organic synthesis, it is important to develop metalloenzyme-inspired polymer-supported metal catalysts. Here, we present a highly active, reusable, self-assembled catalyst of poly(imidazole-acrylamide) and palladium species inspired by metalloenzymes and apply our convolution methodology to the preparation of polymeric metal catalysts. Thus, a metalloenzyme-inspired polymeric imidazole Pd catalyst (MEPI-Pd) was readily prepared by the coordinative convolution of (NH4)2PdCl4 and poly[(N-vinylimidazole)-co-(N-isopropylacrylamide)5] in a methanol–water solution at 80 °C for 30 min. SEM observation revealed that MEPI-Pd has a globular-aggregated, self-assembled structure. TEM observation and XPS and EDX analyses indicated that PdCl2 and Pd(0) nanoparticles were uniformly dispersed in MEPI-Pd. ME...

Journal ArticleDOI
TL;DR: Graphene supported palladium (Pd) catalyst has been prepared using the conventional impregnation and hydrogen reduction method in this article, and the density functional theory (DFT) study and the catalyst characterization using Raman and X-ray photoelectron spectroscopy confirm that oxygen containing groups play an important role in stabilizing Pd clusters on graphene.
Abstract: Graphene supported palladium (Pd) catalyst has been prepared using the conventional impregnation and hydrogen reduction method. Highly dispersed nano particles are formed on the support graphene. The density functional theory (DFT) study and the catalyst characterization using Raman and X-ray photoelectron spectroscopy confirm that the oxygen containing groups play an important role in stabilizing Pd clusters on graphene. The first layer of the metal particle mainly presents as PdO x . The graphene supported Pd catalyst shows superior catalytic activity and high stability for CO oxidation. The kinetic studies indicate that CO oxidation over the graphene supported Pd catalyst follows the Langmuir–Hinshelwood (L–H) mechanism.

Journal ArticleDOI
TL;DR: An aqueous ratiometric ESIPT sensor with a 87 nM (15.4 ppb) detection limit was successfully synthesized and applied for detection of all oxidation states of palladium species.

Journal ArticleDOI
TL;DR: Palladium's pore cousin: a facile approach is described for the size-controlled preparation of porous single-crystalline Pd nanoparticles that exhibit size-independent catalytic activities for the Suzuki coupling and are more active than commercial Pd/C catalysts.
Abstract: Palladium's pore cousin: a facile approach is described for the size-controlled preparation of porous single-crystalline Pd nanoparticles. These porous Pd nanoparticles exhibit size-independent catalytic activities for the Suzuki coupling and are more active than commercial Pd/C catalysts.


Journal ArticleDOI
TL;DR: These monodisperse palladium (Pd) nanoparticles on reduced graphene oxide (RGO) surfaces were successfully prepared by a "wet" and "clean" method in aqueous solution and exhibited catalytic activity in hydrogen generation from the hydrolysis of ammonia borane.
Abstract: In this study, monodisperse palladium (Pd) nanoparticles on reduced graphene oxide (RGO) surfaces were successfully prepared by a “wet” and “clean” method in aqueous solution. Without any surface treatment, Pd nanoparticles are firmly attached to the RGO sheets. These RGO/Pd nanocomposites exhibited catalytic activity in hydrogen generation from the hydrolysis of ammonia borane (AB). Their hydrolysis completion time and activation energy were 12.5 min and 51 ± 1 kJ mol−1, respectively, which were comparable to the best Pd-based catalyst reported. The TOF values (mol of H2 × (mol of catalyst × min)−1) of RGO/Pd is 6.25, which appears to be one of the best catalysts reported so far. We also obtained a 11B NMR spectrum to investigate the mechanism of this catalytic hydrolysis process. This simple and straightforward method is of significance for the facile preparation of metal nanocatalysts with high catalytic activity on proper supporting materials.

Journal ArticleDOI
TL;DR: The phase purity of the synthesized palladium nanoparticles was investigated through X-Ray Diffraction (XRD) analysis and the obtained pattern was compared with JCPDS data as mentioned in this paper.
Abstract: Palladium (Pd) nanoparticles were synthesized using protein rich soybean leaf extract based biological process. Reduction of palladium ions by soybean leaf extract was examined by UV-visible spectroscopic technique. It was believed that the proteins and some of the amino acids that are exist in soybean leaf extracts were actively involved in the reduction of palladium ions. Further it was confirmed by Fourier transformations infrared spectroscopic (FTIR) analysis. These amino acids are not only involving in the reduction of palladium ions but also acting as surfactants that inhibits the rapid agglomeration. The phase purity of the synthesized palladium nanoparticles was investigated through X-Ray Diffraction (XRD) analysis and the obtained pattern was compared with JCPDS data. Transmission electron microscopic (TEM) images of the palladium particles were recorded and the particle size was found to be ~15 nm.

Journal ArticleDOI
TL;DR: Asymmetric catalysis with palladium, phosphines, and unactivated C(Sp(3))-H Bonds shows cooperative effects in the context of quaternary Stereogenic Centers and asymmetric Hydrogenation.
Abstract: Keywords: asymmetric catalysis ; C?H activation ; cooperative effects ; palladium ; phosphines ; C-H Activation ; Quaternary Stereogenic Centers ; Intramolecular Alkane Arylation ; Unactivated C(Sp(3))-H Bonds ; Asymmetric Hydrogenation ; Mild Conditions ; Methyl-Groups ; Ligands ; Derivatives ; Indolines Reference EPFL-ARTICLE-176461doi:10.1002/anie.201108511View record in Web of Science Record created on 2012-04-26, modified on 2017-12-03

Journal ArticleDOI
TL;DR: A photoredox palladium/iridium-catalyzed C-H arylation with diaryliodonium reagents that is believed to proceed via an 'ionic' 2e- pathway and requires a much higher reaction temperature (100 ºC).
Abstract: This paper describes a photoredox palladium/iridium-catalyzed C-H arylation with diaryliodonium reagents. Details of the reaction optimization, substrate scope, and mechanism are presented along with a comparison to a related method in which aryldiazonium salts are used in place of diaryliodonium reagents. The unprecedentedly mild reaction conditions (25 oC in methanol), the requirement for light and a photocatalyst, the inhibitory effect of radical scavengers, and the observed chemoselectivity trends are all consistent with a radical-thermal reaction with diaryliodonium reagents that is believed to proceed via an 'ionic' 2e- pathway and requires a much higher reaction temperature (100 oC).

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of water/hydroxyl inhibition on catalytic deactivation of catalysts supported on metal oxides and showed that the presence of water on the catalysts impedes the catalytic combustion reaction by hindering oxygen mobility on the support.
Abstract: Palladium-based catalysts supported on metal oxides are attractive for methane combustion at low temperature. However, at temperatures below 450 °C, their tendency to deactivate hinders their usefulness. Catalytic deactivation in this temperature regime has been attributed to a water/hydroxyl inhibition effect. We investigated this effect to better understand the mechanism for catalytic deactivation. Comparative in situ FTIR transmission spectroscopy experiments at 325 °C revealed that hydroxyl accumulation occurs on the oxide supports during catalytic methane combustion and deactivation. The water/hydroxyl accumulation on the support is slow to desorb at this temperature. In light of our recent finding that oxygen from the support is utilized in the methane combustion process, we propose that hydroxyl/water accumulation on the support impedes the catalytic combustion reaction by hindering oxygen mobility on the support. We support this hypothesis by demonstrating that the presence of water on the catalys...

Journal ArticleDOI
TL;DR: The decarboxylative C H bond arylation of thiophenes catalyzed by a Pd(OAc)2/Ag2CO3 system is reported, which offers a new synthetic strategy in synthesis and the challenges posed by this promising process remain.
Abstract: A combination of Ag2CO3 with Pd(O-Ac)2/PCy3 or Pd(O-CO-CF3)2/PCy3 is found to efficiently promote the title reaction.


Journal ArticleDOI
TL;DR: In this article, the authors describe self-assemblies formed by the combination of a variety of palladium(II) components and ligands ranging from bi-to polydentate.

Journal ArticleDOI
TL;DR: In this article, a magnetic nanoparticle-supported palladium catalyst has been developed for the Suzuki, Sonogashira and Heck reactions, and the performance of the magnetic separation of the catalyst was very efficient, and it was possible to recover and reuse it at least eight times without significant loss of its catalytic activity.
Abstract: A highly efficient, air- and moisture-stable and easily recoverable magnetic nanoparticle-supported palladium catalyst has been developed for the Suzuki, Sonogashira and Heck reactions. A wide range of substrates was coupled successfully under aerobic conditions. In particular, the performance of the magnetic separation of the catalyst was very efficient, and it is possible to recover and reuse it at least eight times without significant loss of its catalytic activity.

Journal ArticleDOI
TL;DR: In this paper, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces.
Abstract: The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

Journal ArticleDOI
TL;DR: In this article, an efficient chemical reduction method was developed to produce highly active palladium (Pd) nanoparticles in polyethylene glycol (PEG) with no other stabilizer and the as-prepared Pd/PEG catalyst demonstrated a remarkable catalytic activity toward hydrogenation of both styrene and nitrobenzene under mild conditions.